Can someone explain Henrik Jensen's dipole diffusion approximation method? I have read jensen's papers but this is the part that I can't understand very well. He seems to be placing two theoretical lights, one above and one below the surface, taking measurements from these lights to the surface and multiplies each of these distances by the extinction coefficient, the results of which go into the main equation for determining the light contribution of a sample point for a specific shading point, which is in itself a pretty complex, expensive equation to solve. What are these two lights set up in a dipole configuration doing for you? Are they set up at the point being shaded or at the irradiance sampling point?
I see other implementations out there that are simpler, like the 2003 Pixar PrMan Siggraph paper which just uses a smoothstep(), comparing the distance between the sample point and the point being shaded to the scattering distance.
What does the "diffuse dipole approximation" give you?